This invention relates to packet data radio communication systems, and particularly to such systems where there is a need at a radio receiver to receive incoming relatively short bursts of data.
In a land mobile data communication system there typically will be a number of base stations at fixed locations covering predetermined geographic areas and a much larger number of mobile stations in vehicles that travel within and between those geographic areas. Where packet data communication or digital control signalling is used, base stations will, in effect, broadcast to the mobile stations, and the mobile stations will transmit signals to the base stations in bursts, as required by the circumstances. When signal burst duration is minimized to maximize radio channel capacity, each signal burst tends to require controlled amplitude envelope rise and fall times, or "ramp up" and "ramp down" time, and typically contains various control symbols as well as information data packets.
Ordinarily, synchronization for each such burst must be performed using only the waveform of the individual burst. Thence, each such burst conventionally includes a preamble having a symbol timing synchronization word ("STSW"), and a frame timing synchronization word ("FTSW") for modulation symbol detection and data block or frame synchronization. However, this approach requires many preamble symbols which, by consuming channel time that could be used for transmitting information, i.e., the preamble symbols are "overhead" which reduces channel efficiency, especially in bursty communications.
Therefore, there is a need for a method and apparatus for synchronizing bursts of incoming data that does not require high overhead and, thereby, provides greater channel efficiency, and wherein symbol synchronization is derived from the information symbols rather than from the use of a special preamble.